Authentication of a user's identity involves verifying a user is who he or she represents himself or herself to be or has credentials, typically for accessing data or a service. Authentication is particularly useful in computer security to prevent a user from accessing data available via a computer system but for which the user does not have access permission. Biometric authentication techniques may be used. Authentication may be desired on a continuous basis and in a manner which does not interrupt the user's activity in interfacing with an application, computer system or machine controlled by a computer system. For example, distraction caused by interrupting a user to re-enter a password or put his or her eye to a retinal scanning device while engaged in an activity is to be avoided.